Chromosomal abnormalities are responsible for a significant number of birth defects, including mental retardation. Abnormalities can appear in the form of chromosomal DNA duplications and deletions, as well in the form as chromosomal aneuploidy, which is the abnormal presence or absence of an entire chromosome. Conditions where an organism has less than, or more than the normal diploid number of chromosomes give rise to a multitude of abnormal characteristics and are responsible for many syndromes. Down's syndrome, or trisomy 21, is the most common example of a chromosomal aneuploidy and involves an extra chromosome 21. Other common chromosomal aneuploidies are trisomy 13, trisomy 18, Turner's syndrome and Klinefelter's syndrome.
The options for the prenatal detection of chromosomal abnormalities are mainly limited to invasive methods with a small but finite risk for fetal loss. The most common method for detection of abnormalities is amniocentesis. However, because amniocentesis is an invasive method it is generally performed only on older mothers where the risk of a fetus presenting with chromosomal abnormalities is increased. It would therefore be beneficial to establish non-invasive methods for the diagnosis of fetal chromosomal abnormalities that can be used on larger population of prospective mothers. One such non-invasive method has been described in U.S. Pat. No. 4,874,693, which discloses a method for detecting placental dysfunction indicative of chromosomal abnormalities by monitoring the maternal levels of human chorionic gonadotropin hormone (HCG). However, while this method is non-invasive and can be used to screen prospective mothers of all ages, it does not serve as a diagnostic of the particular chromosomal abnormality present, nor is a guarantee of its presence.
In addition to being invasive at the sample taking step, the existing prenatal diagnosis methods are also time consuming to perform. For example, Geisma-staining is the technique most widely used and requires that the cells be in metaphase or dividing, when the test is preformed. Each chromosome pair stains in a characteristic pattern of light and dark bands. Using this method all of the chromosomes can be individually distinguished and readily reveal the nature of any structural or numerical abnormalities. Geisma-staining does not always detect subtle chromosomal rearrangements. If chromosomal rearrangements are suspected and not detected using this method, further detailed analysis can be done using fluorescent in situ hybridization (FISH) or spectral karyotyping (SKY). Tests results using Geisma-staining can take one to two weeks.
SKY is a technique that paints each of the metaphase chromosomes with a different probe (dye color). Because each chromosome-specific probe emits its own signature wavelength of fluorescence, structural rearrangement are easily seen, and the chromosomes involved can be readily identified. SKY requires that cells be in metaphase therefore results can take one to two weeks.
FISH is a technique that uses a fluorescent probe (dye) that attaches, or hybridizes, to specific individual chromosomes or certain regions of chromosomes. The affected chromosomes or regions fluoresce, or signal, their presence, or lack of, and can be visually analyzed through a fluorescent microscope. FISH is used to identify particular chromosomal rearrangements or rapidly diagnose the existence of an abnormal number of chromosomes. FISH is currently the most rapid diagnosis method of abnormal chromosome numbers. The speed is possible because cells do not need to be in metaphase in order to do the analysis. Results of the test are typically known in two to three days
Thus, there is a need in the art for non-invasive prenatal diagnostic methods that can rapidly and accurately help determine the presence and the type of chromosomal aberrations.